The present invention relates to a surface acoustic wave filter and more particularly to a surface acoustic wave filter having a plurality of surface acoustic wave propagation paths formed in parallel.
Recently, in mobile communication systems such as portable telephones, a new digital system referred to as a CDMA (Code Division Multiple Access) system is being adopted in addition to a TDMA (Time Division Multiple Access) system.
IF (Intermediate Frequency) filters used for the CDMA system are required to exhibit a frequency characteristic curve with remarkably excellent shape factor as compared with conventional portable telephone systems. Here the shape factor is the ratio of bandwidths (a first bandwidth and a second bandwidth) at two attenuation amounts, i.e., the second bandwidth/the first bandwidth, as shown in FIG. 32. The bandwidths at the two attenuation amounts are, for example, a 3-dB bandwidth and a 10-dB bandwidth. The closer to one the ratio of the bandwidths at the two attenuation amounts is, the more excellent the shape factor is. Thus, excellent shape factor indicates that a filter characteristic curve shows a sharp change.
FIG. 31 shows a transversal filter which is one of surface acoustic waver filters currently in use. This filter is composed of an IDT (interdigital transducer) for inputting signals on one side and an IDT for outputting signals on another side.
The IDT on the right in FIG. 31 is comprised of regular electrodes whose electrode fingers, extending upward and downward, have a fixed length while the IDT on the left is comprised of electrodes weighted by apodization whose electrode fingers differ in length according to a certain rule.
The shape factor of frequency characteristic curves of surface acoustic waver filters has been improved by weighting IDTs like such apodization of electrodes for weighting.
However, for realizing a surface acoustic wave filter having a sufficiently good shape factor by weighting an IDT as by weighting electrodes by apodization, a huge number of electrode pairs are required. That gives rise to a problem that the length in a surface acoustic wave propagation direction (the right and left direction in FIG. 31) increases. For satisfying the portability and size reduction of portable telephones and the like, surface acoustic wave filters also need to be smaller. However, increase in the size of the surface acoustic wave filters for improving the shape factor contradicts the need for their size reduction.
Therefore, it is difficult to realize a surface acoustic wave filter which has characteristics with excellent shape factor and is small in size by improving the shape factor through the conventional weighting of IDTs as shown in FIG. 31.
The present invention is characterized by providing a surface acoustic wave filter whose length in the surface acoustic wave propagation direction is small and which has a frequency characteristic with excellent shape factor by use of an IDT having a small number of electrode fingers.
The present invention provides a surface acoustic wave filter characterized in that it comprises a piezoelectric substrate, a plurality of surface acoustic wave propagation paths arranged in parallel to each other on the piezoelectric substrate and a coupler for coupling the surface acoustic wave propagation paths, an input interdigital transducer and a first reflector are disposed on at least one of the surface acoustic wave propagation paths with intervention of the coupler, and an output interdigital transducer and a second reflector are disposed on at least one of the other surface acoustic wave propagation paths with intervention of the coupler.
By providing this construction, it is possible to reduce the length of the surface acoustic wave filter in the surface acoustic wave propagation direction as compared with conventional ones and to provide a surface acoustic wave filter having a frequency characteristic curve with excellent shape factor.
The present invention also provides a surface acoustic wave filter characterized in that the input interdigital transducer and the output interdigital transducer are disposed side by side in a direction perpendicular to a surface acoustic wave propagation direction, and the length in the surface acoustic wave propagation direction of a portion in which the input interdigital transducer and the output interdigital transducer overlay each other in said perpendicular direction is shorter than the length in the surface acoustic wave propagation direction of the shorter one of the input interdigital transducer and output interdigital transducer.
With this construction, isolation between the input and output IDTs of the surface acoustic wave filter can be improved and the degree of suppression out of a passband can be improved.
Further, the present invention provides a surface acoustic wave filter characterized in that an earth electrode is formed on the piezoelectric substrate between the aforesaid input interdigital transducer and the aforesaid output interdigital transducer.
By thus forming the earth electrode, the isolation between input and output can be further improved.